Objective Emerging evidence indicates that longer SARS-CoV-2 vaccine dosing intervals results in an enhanced immune response. However, the optimal vaccine dosing interval for achieving maximum immunogenicity is unclear.

Methods This study included samples from adult paramedics in Canada who received two doses of either BNT162b2 or mRNA-1273 vaccines and provided blood samples 6 months (170 to 190 days) after the first vaccine dose. The main exposure variable was vaccine dosing interval (days), categorized as “short” (first quartile), “moderate” (second quartile), “long” (third quartile), and “longest” interval (fourth quartile). The primary outcome was total spike antibody concentrations, measured using the Elecsys SARS-CoV-2 total antibody assay. Secondary outcomes included: spike and RBD IgG antibody concentrations, and inhibition of angiotensin-converting enzyme 2 (ACE-2) binding to wild-type spike protein and several different Delta variant spike proteins. We fit a multiple log-linear regression model to investigate the association between vaccine dosing intervals and the antibody concentrations.

Results A total of 564 adult paramedics (mean age 40 years, SD=10) were included. Compared to “short interval” (≤30 days), higher dosing interval quartiles (moderate: 31-38 days; long: 39-73 days and longest: ≥74 days) were all associated with increased Elescys spike total antibody concentration. Compared to the short interval, “long” and “longest” interval quartiles were associated with higher spike and RBD IgG antibody concentrations. Similarly, increasing dosing intervals increased inhibition of ACE-2 binding to viral spike protein, regardless of the vaccine type.

Conclusion Increased mRNA vaccine dosing intervals longer than 30 days result in higher levels of circulating antibodies and viral neutralization when assessed at 6 months.

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